Moles Melanocytic Nevi
Nevi are benign skin tumors composed of melanocyte-derived nevus cells. Melanocytes originate from neural crest tissue. Melanocytic nevi are classified by age of onset and by the location and arrangement of nevus cells within the skin. Melanocytic nevi are composed of organized clusters of nevus cells arranged at various levels in the skin. Nevi are present on humans and equally common among males and females. Most adults have between 12 and 20 nevi. Larger numbers of nevi may be a familial trait. Nevi are usually asymptomatic though may on occasion be irritated by clothing or external trauma.
Nevi present at birth or that appear during infancy are termed congenital nevi. The appearance of newly acquired nevi reaches a peak during adolescence. Fewer nevi are acquired after age 30. Sun exposure appears to be a stimulus for cell growth of nevi as most acquired nevi appear on sun-exposed skin. Existing nevi may increase in size and become more heavily pigmented during puberty or during pregnancy. Nevi appearing after age 30 should be regarded as suspicious as should acquired nevi appearing in sun-protected areas.
Evolution of Lesions
Acquired nevi first appear as flat, round, uniformly colored papules. During this growth phase, nevi expand laterally while remaining flat and symmetric. Nevi may be slightly darker in color and slightly raised in the center, and may remain stable in size and appearance for several years. Over many years, nevi continue to become more elevated and uniformly lighter in color. Eventually the nevus appears as a skin-colored papule or may completely disappear in older years. Residual nevi are rare after age 70.